"Unable to create version backup"
Google Drive is very likely the reason in this case, as it has to lock access to the file while syncing it with the Cloud. But it can also happen (and does happen) to me when a file is located on the server, and the server is creating Shadow Copies of it. In our company the server is setup this way, every hour changed files will be backed up, so the likeliness of saving over just that one blend file which is currently being backed up is not that small.

What I noticed then is that on the Server a file with an '@' at the end will be left behind. I have to manually delete it, before I can save my blend file again, if not Blender will continue to complain about it being there.